64 CHROMATOGRAPHY
Alternatives to helium A
In the face of rising prices for helium, Ed Connor reveals the more cost-effective options for gas chromatography.
lthough there is now less concern about its availability, the price
of helium is set to continue to increase, meaning that cost- effective alternatives are still attractive to analytical labs.
Many of the labs affected by the helium shortage of 2012-2013 have found that they are now able to source helium readily, although they have seen stark increases in price over the past two years.
One direct consequence of the helium shortage was the increase in information made available to chromatographers about alternative carrier gases and the resulting increase in the uptake of nitrogen and hydrogen as alternatives, owing to a change in perception relating to these gases.
Nitrogen is seen as a slow gas and is often overlooked as an alternative to helium, when in fact its use would be perfectly valid in a number of GC analyses. With a low optimal linear velocity of 8-14 cms-1 for nitrogen, compared with 25-33 cms-1
sample throughput, however it is not always possible to analyse a sample at a higher linear velocity because of lack of peak resolution. Matching the linear velocity of hydrogen to that of helium should mean like-for-like analysis, with slightly improved carrier efficiency.
compressed air by pressure swing adsorption using a carbon
for helium (Fig. 1),
analysis times will be increased if the analyst wants to maintain optimal performance. However, if there is enough resolution between peaks, it is possible to run samples at a higher average linear velocity. Tis means sacrificing some theoretical plates, which in practical terms means broadened peaks.
Hydrogen, on the other hand, is more efficient than helium at higher linear velocities, with an optimal linear velocity range of 38-45 cms-1
, meaning that
Table 1. (Right) Peak areas, theoretical plate count, resolution and peak width of Decane, Undecane and Dodecane run with helium, nitrogen and hydrogen carrier gases.
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efficiency is improved compared with helium.
Tis improved efficiency at higher linear velocities potentially allows for increased
Carrier Gas Decane Peak Area Undecane Peak Area Dodecane Peak Area Decane Theoretical Plates Undecane Theoretical Plates Dodecane Theoretical Plates Decane Resolution Undecane Resolution Dodecane Resolution Decane Width Undecane Width Dodecane Width
Helium 54376.5 54300.6 53348.4 49352 74117 102110 13.715 21.542 38.533 0.140 0.131 0.154
Nitrogen 53528.2 53250.6 52592.5 37802 48585 57467 11.409 6.973 29.353 0.126 0.177 0.196
Hydrogen 52180.2 52498.7 52025.9 44631 68726 101117 12.987 20.497 37.642 0.135 0.149 0.159
One concern that people have about working with hydrogen is its flammability, and placing hydrogen cylinders in the lab is a potential health and safety risk. Terefore the use of generators to produce hydrogen, and indeed nitrogen, is a cost-effective and safe method of gas production within the lab. Hydrogen is produced by the electrolysis of deionised water and is supplied to the GC on-demand. Te hydrogen generator contains only a low volume of hydrogen at much lower pressure than cylinders, whilst being able to produce enough gas to supply a whole lab. Nitrogen is produced by the removal of oxygen, CO2
and hydrocarbons from
molecular sieve material.
A three-component alkane standard was run using a Shimadzu 2010 GC with a Restek RTX-1 column (30 m x 0.25 mm x 0.25 µm) using cylinder helium, and hydrogen and nitrogen produced by gas generators as the carrier gases. Te samples were run
isothermally (170°C) using the three carrier gases at the same linear velocity (37.5 cms-1
) to
look at the effect on peak area, peak width, theoretical plate count and resolution.
Fig. 2 shows that the results obtained from this short run are very similar regardless of the carrier gas used.
Hydrogen and helium give almost identical results, with the peaks in the nitrogen run showing a little band broadening and although the linear velocity of all three carrier gases was set to 37.5 cms-1
,
the alkane peaks run with nitrogen carrier are slightly delayed compared with the peaks obtained using hydrogen and helium.
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